Substantial development has taken place in connection with circuit protection devices of a type including resistivity-temperature sensitive, conductive polymeric or ceramic material. In the presence of sufficiently low currents, the resistive heating of the conductive polymer is inadequate to disturb its low resistance state equilibrium. Alternatively, overcurrent or overtemperature conditions tend to cause the conductive polymer or ceramic material to self-heat at a rate faster than its thermal dissipation rate. At the same time, resistivity of the conductive polymer rises sharply and causes a reduction of current passing through the said material. As discussed in greater detail below, various forms of the polymer are available for regulating different electrical parameters such as power or wattage.
In any event, electrical conductivity of the polymeric material can be increased significantly by incorporation of adequate conductive filler, for example, carbon black or metal particles. The electrical resistivity of the conductive polymer is determined by the type of conductive particles and the volumetric ratio of total conductive particles in the polymer.
Particular interest has developed in compositions of which resistivity exhibit positive temperature characteristics, commonly referred to as "PTC". These materials exhibit a very rapid increase in resistivity over a selected temperature range. The temperature at which the resistance commences to increase sharply is often designated the switching or autotherming temperature. On reaching that temperature, the conductive polymer exhibits an anomalous change in resistance which, for practical purposes, electrically switches off a heater or other device with which the conductive polymer is associated.
The conductivity of many ceramics is significantly influenced by both ionic and electronic defects. Many oxides are metallic conductors and exhibiting positive-temperature-coefficient (PTC). The perovskite- and rutile-structure oxides are the most frequently studied. Examples of PTC ceramics are doped barium titanate, BaTiO.sub.3 with small amount of CuO. Polycrystalline ZnO with Bi.sub.2 O.sub.3 and other additives has nonohmic conduction and is used as a voltage-dependent resistor. Reference of ceramics as electrical materials can be found in the Encyclopedia of Chemical Technology, Vol. 5 pp 290 (by H. K. Bowen) 1984 John Wiley Son, Inc. N.Y.
Additional characteristics concerning ceramic material suitable for use within the present invention are disclosed for example in W. D. Kingery, H. K. Bowen and D. R. Uhlman Introduction to Ceramics, Second Addition, John Wiley & Son, Inc., N.Y. 1976 and U.S. Pat. No. 4,486,651 issued Dec. 4, 1984 to Kinya Atsumi, et al and assigned to Nippon Soken, Inc. and entitled Ceramic Heater. Those references are accordingly incorporated as though set forth herein in their entirety.
Another well known type of conductive polymer is commonly referred to as a constant wattage type or "ZTC". These materials have the ability to regulate overall power or wattage in a circuit rather than current as with the PTC materials. The relatively constant resistance in the temperature range of normal operation results in a constant wattage heating behavior.
Rather than including a detailed discussion of such materials which are well known in the prior art, reference is made below to a number of prior U.S. patents, all of which are incorporated herein by reference as though set forth in their entirety in order to assure a proper understanding of the various possibilities for conductive polymers as temperature responsive electrical regulating components in connection with the present invention.
For example, it has been proposed to use devices comprising conductive polymer for protecting circuits against fault conditions arising from excessive temperatures and/or electrical currents. For example, U.S. Pat. No. 2,978,665 issued Apr. 4, 1961 to Vernet, et al disclosed the use of such materials in devices for regulating electrical current. Further developments in such devices were disclosed, for example, by U.S. Pat. Nos. 3,243,753 and 3,351,882 issued respectively on Mar. 29, 1966 and Nov. 7, 1967 to Kohler.
Subsequently, U.S. Pat. No. 4,352,083 issued Sept. 28, 1982 to Middleman, et al further referred to the use of such materials in circuit protection devices. That patent was also of interest in that it included a substantial bibliography of U.S. patents and other publications relating particularly to the use of conductive polymers in such circuit protection devices. The Middleman, et al patent and the other patents referred to therein are incorporated herein as though set forth in their entirety in order to assure an understanding of the capabilities and versatility of such materials.
In an overview of uses for conductive polymers in circuit protection devices, a number of problems have tended to limit their use in circuit protection devices or to eliminate them entirely from consideration in the past.
Initially, it has been found that, when conductive polymers are employed as overcurrent protectors, resistivity of the conductive polymer is not uniformly stable with respect to mechanical stress and temperature cycling. For example, a number of thermal cycles, possibly ten to fifteen cycles, near the melting point of the conductive polymer may drastically reduce the autotherm height or level for the material. This will obviously jeopardize the ability and reliability of the conductive polymer to provide circuit protection. Furthermore, the reduction of the autotherm height may also lead to thermal runaway of the conductive polymer which in turn will result in flame failure for the device including burning of the conductive polymer.
A second problem arising from the use of conductive polymers as resistors in such devices resides in the high autotherm or trip temperature characteristic of most high performance polymers. For example, most high performance polymers such as the general class of fluoropolymers exhibit very high melting points or trip points typically 250.degree. Centigrade. Thus, even with the desirable properties of such high performance polymers, their use tends to be excluded in most electronic circuits or systems.
Yet another problem which has been found to exist in connection with the use of such conductive polymers is their tendency to exhibit a small residue current rather than a true open circuit after reaching a predetermined set temperature or trip point. Because of this characteristic, a limited amount of current, depending on the aging and stability of the composite, tends to flow within the circuit even above the predetermined set temperature or trip point. This characteristic minimizes the desirability of such devices in certain applications and even totally eliminates the possibility of their use in other applications such as primary circuit protection where a true open circuit is required above the predetermined set temperature or trip point.
Other limitations or problems have been found to exist in connection with conductive polymers and ceramics such as those discussed above. The problems summarized immediately above are believed to be particularly representative of other undesirable characteristics which may be encountered in the use of conductive polymers and ceramics. In any event, there has been found to remain a need for circuit protection devices including conductive material while being able to minimize or overcome problems such as those outlined above.